Lakes and landscapes preserve unique records of past climate conditions. LLO researchers are working to develop a broad range of techniques for decoding the signals preserved in lake sediments and landscapes. These approaches are being used to evaluate climate history in East Africa, Central Asia, and North America.

Bear Lake, Utah-Idaho

Bear Lake is a medium-sized (10 x 30 km) lake that occupies a relatively simple half graben at the boundary between the Colorado Plateau and the Basin and Range. It has many advantages for recording the history of climate change in the western United States: it is a relatively deep marl lake in a glaciated drainage and it has a nearly continuous sedimentary sequence extending more than 250,000 years. A cooperative project among several Universities and the U.S. Geological Survey has been investigating the paleolimnologic and paleoclimate record of the lake. The record turns out to be very complex because groundwater discharge from karst terrain, major hydrological changes related to the changing course of the Bear River, and episodic tectonic activity. Yet this complexity is yielding unexpected insights into the history of the lake and its environment.

Lake Edward, East Africa

Lake Edward is located in the western arm of the East African Rift Valley, situated on the equator between the Democratic Republic of the Congo and Uganda. The lake is in a key location for paleoclimate study, lying at the eastern boundary of the vast, tropical Congo Basin. We completed a reconnaissance survey of Lake Edward in June 1996 with a high-resolution seismic reflection profiling system and collected four cores of 5-8 m length for paleoclimatic study. The cores have undergone extensive paleoclimate analysis, providing valuable new insights into the trends and pulse of African tropical climate.

Publications:

Russell, J. M. and Johnson, T. C., in press. The Water Balance and Stable Isotope Hydrology of Lake Edward, Uganda-Congo. Journal of Great Lakes Research.

Issyk-Kul, Central Asia

Faculty, staff and graduate students have now completed four expeditions on Issyk-Kul, Kyrgyzstan, in collaboration with Ken Rasmussen, from the Smithsonian Institution, and Vladimir Romanovsky, from the Kyrgyz Institute of Hydroenergetics and Water Problems. Issyk-Kul is the eleventh largest lake in the world by volume (1730 km3) and the fifth deepest (668 m). It is a closed-basin lake at the junction of the Siberian High and the Indian Low, two pressure cells that control atmospheric circulation over central Asia. During the 1997 field expedition, piston cores were collected for paleoclimate studies. The sediment from the cores were analyzed for several paleoclimate proxies, including the stable isotopic and trace element composition of ostracodes. The data show relatively wet conditions around 6000 - 8000 yr before present, followed by dry conditions leading to a closed-basin lake and progressively higher salinities for much of the subsequent 6000 years (Ricketts et al., 2001). Multicores were collected from the lake in 1999 to focus on the most recently deposited sediment and piston cores were collected from the lake in 2000, to extend the record further back in time.

Lake Malawi, East Africa

LLO was funded by the National Science Foundation (NSF) to recover a high-resolution record of past climate change recorded in the varved sediments of northern Lake Malawi. Lake Malawi is the second largest lake in the East African Rift Valley, and is located between 9 and 14 degrees south latitutde. It is about 650 km long and nearly 700 m deep. Piston cores and multicores were recovered in 1997 and 1998 from the north and central basins of the lake. The cores continue to be analyzed for new records of past climate change in the African tropics. The multi-cores contain a 450-year long record of varve thickness and biogenic silica concentrations that show strong evidence for ENSO scale climate variability as well as the impact of the Little Ice Age on this southern African lake. Longer piston cores demonstrate episodes of varve deposition extending back many thousands of years, showing strong links to climate variability in the Northern Hemisphere on a millennial scale.

The paleoclimate records generated by LLO scientists and their collaborators were instrumental in the development of a drilling program on Lake Malawi that was funded by NSF and the International Continental Drilling Program, and carried out in February – March 2005. We drilled two sites – one in the central basin in a water depth of 600 m, to a depth of 385 m below the lake floor, representing roughly the past 1.5 million years of climate change in the African tropics. The other site was in the north basin, where we triple cored a sediment sequence to a major unconformity that we estimate to be 75,000 years old. Surprisingly, Lake Malawi, Lake Tanganyika, and Lake Bosumtwi in west Africa all show an arid phase at 75 ka that was more severe than the last glacial maximum (Scholz et al., 2005). Malawi was the first of the large lakes in the East African Rift Valley to be drilled for paleoclimate records.

Lake Nicaragua, Central America

The LLO participated in a reconaissance study of Lake Nicaragua in Central America with scientists from the University of Michigan in Ann Arbor in June 1997. Over 500 km of seismic reflection profiles were obtained using a small airgun, and gravity cores and grab samples were collected at several sites to assess the potential of Lake Nicaraguan sediments as an archive of past climate change in Central America. The sediments throughout the lake contain abundant volcanic ash, reflecting the tectonic setting of the region.

Lake Qinghai, China, Drilling Project

The sediments of Lake Qinghai, the largest lake in China, were drilled and cored in 2005 and 2007 in several locations. The drilling operations were funded by the International Continental Drilling Program and the Chinese Academy of Sciences, and analysis of the cores has been supported by NSF. Lake Qinghai is a large shallow lake at 3200 m elevation in the northeast corner of the Tibetan Plateau. Its sediments contain a unique record of climate in this part of Asia, which is affected by the Asian monsoon and the northern hemisphere westerlies. They also provide insights into the timing of some of the tectonic processes that resulted in the growth of the northeast Tibetan Plateau. Steve Colman and Tom Johnson attended the primary planning meeting for this project in Xining, China, in October, 1993, and Colman is now the primary U.S. PI on the research involving analysis of the sedimentary record.

Lake Superior, North America

The sediments and geomorphology of the Lake Superior basin contain a record of deglacial events, including a remarkable varved sequence deposited between 9500 and 11,000 years ago. The varve record, as well as the seismic stratigraphy of late-glacial deposits reflect the history of deglaciation, meltwater discharge from Glacial Lake Agassiz, and changing lake level. LLO scientists are deciphering this history using a variety of geophysical, geochemical, and sedimentological analyses, and linking their observations to the North Atlantic region, where Lake Agassiz discharge is thought to have disrupted thermohaline circulation and the climate of northern Europe. Other research topics are the unusual polygonal pattern superimposed on the postglacial sediments and the effect this has on the pattern of recent sediment accumulation.

Lake Victoria, East Africa

Analyses continue on sediment cores recovered from Lake Victoria as part of the International Decade for the East African Lakes (IDEAL) 1995 and 1996 field seasons. Biogenic silica profiles in the piston cores reflect a strong correlation with oxygen isotope profiles derived from aquatic cellulose. With the paleoclimatological analyses of these sediments cores, results indicate that Lake Victoria, beginning to refill around 14.7 ka after complete desiccation during the last ice age, did not achieve open-basin status until about 11.2 ka, when the Victoria Nile was formed.

Molecular isotopic paleo-studies

New approaches in paleoenvironmental studies utilizing molecular isotopic methods have proven fruitful in many ways. By analyzing the distribution and variability of biomarkers (organic compounds that can be traced to a known biological source) in lake and ocean sediments, we are reconstructing past variations in terrestrial vegetation (e.g., rainforest vs. savannah, C3 vs C4 plant types) and aquatic primary production (e.g. shifts in dominant algal taxa present) in a variety of environments. These vegetation changes can then be related to climate change. Another exciting avenue of molecular isotopic paleoclimate research in which we are involved is the reconstruction of continental paleotemperature and hydrology from lake sediment archives. Specific membrane lipids from crenarchaeota living in the water column have been shown to have a relationship to temperature, and hydrogen isotopes of terrestrial and aquatic biomarkers can be utilized to reconstruct hydrological conditions quantitatively if temperatures are known for a given system. These studies are being carried out in East Africa (Lake Malawi), the USA (Lake Superior and Elk Lake, MN at the headwaters of the Mississippi River), and Mexico (Lago Verde, Los Tuxtlas, Veracruz). Collaborators include J. Sinninghe Damsté, S. Schouten, and E. Hopmans at NIOZ, W. Dean at the USGS, and M. Caballero-Miranda at UNAM.

Neotectonics

LLO scientists use several approaches to neotectonic studies:
In one, Erik Brown utilizes cosmic ray exposure dating of geomorphological features offset by movement on active faults to estimate slip rates of those faults. Earlier work assumed post-glacial ages for surface features, yielding rapid apparent slip rates and supporting the idea that much of Tibet has been extruded eastward out of India's northward path. This work is showing that many of the surface features are significantly older and that slip rates are correspondingly lower. This suggests, instead, that crustal thickening is the main process by which Asia accommodates India's penetration into it.
In another approach, Steve Colman uses seismic stratigraphy to examine patterns and amounts of young faulting and neotectonic deformation in different lake basins.